Relevant Discussion: 91.215, 91.217, 91.135, 91.411, 91.413, 99.13, 121.345, 135.143, AIM 3-2-3, 3-2-4, 4-1-20, 5-6-4, 6-2-2, 6-4-2, FAA-H-8083-16, 8083-25
Everything You Need to Know about Mode C Transponders
Key Takeaways:
- Mode C transponders are generally mandatory for most aircraft operating in Class A, B, and C airspace, within 30 nm of specific airports, and at or above 10,000 feet MSL in the contiguous U.S., with specific exemptions for aircraft without engine-driven electrical systems, balloons, or gliders under certain conditions.
- Transponder Mode C transmissions report pressure altitude (independent of pilot's altimeter setting) and must agree with the barometric altimeter within ±125 feet; the "Ident" button should only be pressed when requested by ATC.
- Specific procedures allow for flying with an inoperative transponder or requesting to operate without one, and altitude encoders used for IFR flight require re-certification every two calendar years.
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